The invention relates to an electric lamp with reflector, comprising:
a reflector having a reflector body with a concave reflecting surface chosen from surfaces with an ellipsoidal and surfaces with a paraboloidal general shape, an optical axis, a focus within the reflector, and a light emission window; PA1 an electric lamp with a lamp vessel which is closed in a vacuumtight manner and in which a linear electric element is present, arranged on the optical axis. PA1 the light emission window has a diameter D.sub.LS and the electric element has an axial dimension L, D.sub.LS /L being greater than 40.
Such an electric lamp with reflector is described in the non-prepublished European Patent Application 93 20 29 51.5.
The known lamp with reflector may be used for projection purposes, such as film or slide projection, but also in projection TV devices. In these devices, as is the case with film or slide projection, a light-transmitting image carrier is present in a plane perpendicular to the optical axis of the reflector, for example, an LCD screen or a DMD (Digital Mirror Device) screen. Such image careers are usually rectangular, for example, with a width/height ratio of 4/3 or 16/9.
It is the aim of the electric lamp with reflector to illuminate the image carrier brightly and uniformly, so that an optical system, which may comprise a projection lens, is capable of displaying the image clearly and evenly on a screen, so that it can be viewed thereon.
The uniformity of illumination, however, may be adversely affected by an inaccurate placement of the electric element. Furthermore, the electric element may change its place, for example, owing to differences in expansion at high operating temperatures, or because a discharge arc acting as the light source has changing points of application on the electrodes.
The brightness of the illumination is adversely affected by the fact that the electric lamp with reflector provides a round illuminated field, whereas the image carrier is rectangular. A portion of the light is accordingly thrown outside the image carrier. This portion is greater in the case of a more elongate image carrier (16/9) than in the case of a carrier with a shape closer to the square shape (4/3).
U.S. Pat. No. 4,021,659 discloses an ellipsoidal reflector for projection purposes with an incandescent lamp just accommodated therein. The lamp has an incandescent body which is positioned axially. The reflecting surface of the reflector has superimposed facets which are arranged both in radial lanes and in circular bands. The reflector has retained its rotationally symmetrical shape owing to this arrangement of the facets, which are trapezium-shaped. The facets are all perpendicular to the radius with their parallel sides. They may have a convex surface. It is the object of the facets to increase the uniformity of the illumination of an illuminated field.
The facets throw enlarged images of the incandescent body superimposed over one another in the second focus. The photographs included in the cited Patent show that an illuminated field of improved homogeneity is obtained, which nevertheless is still patchy and which has a round shape.